This invention relates in part to a method and system for forming and coating a liquid composition on to a solid surface. More particularly, this invention relates to a process and system for coating a solid surface with a liquid polymer composition. The invention also relates to a method for conditioning liquid polymer compositions to optimize the uniformity of the thin films created during the coating process.
Liquid polymer compositions such as photoresists, antireflective coatings, and spin-on dielectrics are commonly used to coat a solid surface. For example, printed circuits presently are formed on the surface of a wafer such as silicon or gallium arsenide under cleanroom conditions. A commonly used process in microelectronic circuit manufacturing includes a step of applying a liquid photoresist onto the wafer surface and spinning the wafer so that the photoresist forms a thin, uniform coating on the wafer surface. The photoresist then is exposed to light through a patterned mask to transfer a circuit pattern from the mask onto the wafer. The exposed resist is developed to form an image of the desired circuit features on the wafer. Areas not coated with the developed photoresist are then processed further to form a semiconductor device.
Wafers having a diameter of 300 mm or larger are now in development. These diameters are larger than the diameters of commercially produced wafers. In order to attain a photoresist coating of the same thickness and uniformity as for the smaller diameter wafers, the 300 mm or larger wafers must be rotated at speeds that produce turbulent gas flow above the wafer. This turbulence may lead to non-uniform coating thickness, particularly adjacent to the edge of the wafer where the rotational speeds are the highest. A photoresist coating having non-uniform thickness is undesirable since it causes undesired variability in photoresist exposure and development.
To limit the turbulence above the wafer and thereby limit non-uniform photoresist coatings, it has been previously proposed to coat the wafer under partial vacuum. Unfortunately, it has been found that when the photoresist is dispensed into such a reduced pressure environment, the dissolved atmospheric gases in the photoresist liquid out gas by bubble formation. Bubble formation is undesirable because bubbles in the photoresist dispense lines and nozzle cause an inaccurate volume of photoresist to be dispensed on the wafer, thereby affecting final film uniformity on the wafer and the repeatability of the film coating from one wafer to the next. Those of ordinary skill in the art recognize that current typical dispense volumes are in the range of about 1 ml to about 10 ml of photoresist. Typically, each ml of resist is approximately 1 gram. Preferably, an approximately 3 ml dispense (3 grams) is used.
Bubbles are undesirable in the photoresist because they interfere with the exposure and development of the photoresist after it is dispensed onto the wafer. This distortion may result in breaks within the lines formed to define the features in the printed circuit. As the desired circuit elements and conductive lines of a printed circuit become smaller, the goal of the wafer manufacturers, even small bubbles may interfere with accurate transfer of lithographic patterns onto the wafer; generating defects in the lines used to define the printed circuit.
In the spin-coat process, an aliquot of the liquid composition, like a photoresist, is dispensed onto a stationary or slowly rotating substrate. The liquid composition contains solids at least partially dissolved in a volatile solvent. Such liquid compositions are typically true solutions; however, they can also include colloidal dispersions or suspensions. After the photoresist is dispensed onto the substrate, the substrate is rotated at speeds ranging from 1,000 to 7,000 rpm. During this high-speed spin up step, centrifugal forces spread the liquid composition across the substrate. Concurrent with this spin up step, the volatile solvent from the liquid composition evaporates. The result is a thin film of solid material deposited on the substrate. Uniform evaporation of solvent from the liquid composition aids in the formation of a uniform thin film.
In an optimized spin-coat process, the thickness of the deposited film is directly proportional to the viscosity and solvent fraction in the liquid composition deposited on the substrate. In addition, the final film thickness is proportional to the inverse square root of the final spin speed of the substrate. Lastly, optimum film uniformity from the spin-coat process occurs when the substrate is rotated at the highest possible spin speed at a constant temperature. The viscosity of the liquid composition changes with fluctuations in the temperature of the liquid. Accordingly, photoresist temperature is controlled to within xc2x11xc2x0 C. to minimize temperature-related changes in viscosity and hence film thickness.
In a semiconductor spin-coat process, a silicon wafer is typically the substrate and a typical liquid composition is a photoresist. In this application a highly uniform coating of the photoresist film across the wafer is required to achieve accurate transfer of lithographic patterns of the desired circuit features to the wafer. A uniformity of from about 5 to about 100 angstroms is currently needed for deposited photoresist films. The typical thickness of the deposited film ranges from about 0.5 to about 2 xcexcm. While typical substrates currently being produced are about 200 mm in diameter, 300 mm and 450 mm diameter substrates are considered to be the choice for future manufacturing of semiconductors to decrease manufacturing costs and to increase die yields.
Besides accurate control of the viscosity and solvent fraction of the liquid composition, it is also currently necessary to have laminar flow conditions above the rotating substrate in order to achieve optimum final film uniformity.
When interaction between the rotating substrate and the gases above the substrate occurs, turbulent flow conditions can be produced in the gas above the substrate. A dimensionless number called the Reynolds Number, Re, can be used to characterize the flow conditions above the substrate. When the Reynolds Number is below 3xc3x97105 the flow is laminar, and when it is higher turbulent conditions exist. The Reynolds Number can be calculated using the equation below:   Re  =                                                        (                                                (                                      substrate                    ⁢                                          xe2x80x83                                        ⁢                    diameter                    ⁢                                          xe2x80x83                                        ⁢                                          (                      mm                      )                                                        )                                /                                  (                                      2                    *                    1000                                    )                                            )                        2                                                            (                          2              ⁢                              xe2x80x83                            ⁢              π              *                              (                                  rotation                  ⁢                                      xe2x80x83                                    ⁢                  per                  ⁢                                      xe2x80x83                                    ⁢                                      minute                    /                    60                                                  )                                      )                                      (              kinematic        ⁢                  xe2x80x83                ⁢        viscosity        ⁢                  xe2x80x83                ⁢                  (                                    m              2                        /            sec                    )                    )      
Using this equation, the Reynolds Number for a 300-mm substrate rotating at 4,000 rpm in a helium environment (kinematic viscosity 0.000123 m2/sec) would be 7.7xc3x97104.
The presence of turbulent conditions in the gas environment overlying the substrate results in a non-uniform evaporation rate of solvent from the liquid composition during the spin up cycle. As stated above, non-uniform evaporation decreases the final film uniformity across the wafer. As can be determined from the equation, lower Reynolds Numbers can be currently achieved by using high kinematic viscosity gases, lower spin speeds, or smaller substrates.
Gases like helium, hydrogen, and neon have a high kinematic viscosity when compared to air or nitrogen gas. The kinematic viscosity is defined as gas viscosity divided by the gas density. The kinematic viscosity of gases increases with increasing temperature or independently increases with decreasing gas pressure. Thus, reduced pressures and higher temperatures can be used to further reduce the turbulence above a spinning substrate. In a spin-coat process, the combination of reduced pressure, higher gas temperature, and high kinematic viscosity gas will give the lowest Reynolds Number for the environment above the substrate.
In the spin-coat process, a majority of the liquid composition dispensed onto the substrate is not used to coat the substrate. It is sloughed off the substrate and onto the chamber wall because of the centrifugal forces acting on the liquid as the substrate is rotated. As liquid compositions such as photoresist and spin-on dielectrics are very expensive, it follows that the cost of the manufacturing process can be reduced by minimizing the amount of liquid dispensed onto the substrate that is wasted. An accurate and repeatable volume of liquid solution must be dispensed onto the substrate in order to 1) completely coat the substrate; 2) to minimize the waste of these materials; 3) to insure that the thickness of the layer is consistent on the wafer and from wafer to wafer; and 4) to insure that the thickness of the film on the wafer is the desired thickness.
For spin coating in a reduced pressure environment, it is important to control the evaporation rate of solvent from the liquid composition. It is also necessary to control all the forces acting on the liquid so that accurate dispenses of the liquid can be made onto the substrate. In these respects it is very important to accurately control the pressure of the dispense environment.
In a reduced pressure environment, bubbles of gas will form in a liquid composition in both the liquid dispense lines, dispense nozzle(s), and in an aliquot of the liquid dispensed onto the substrate. The formation of bubbles in the liquid is due to outgassing of dissolved gases present in the liquid composition. The presence of bubbles in dispense lines and nozzle due to outgassing will cause inaccurate dispense and spitting of the liquid composition to occur and will effect final film thickness and uniformity.
Prior art means of removing gases and bubbles from liquids, like sparging and vacuum degassing, are effective, but result in uncontrolled loss of solvent from the liquid. Loss of solvent changes the fraction of solvent in the liquid composition and changes the viscosity of the liquid. As the prior art did not recognize the benefit of controlling the loss of solvent, spin-coat processes used in controlled atmosphere environments have undesired differences in film thickness and uniformity. Such liquids cannot be deposited on the substrate during the spin-coat process with the promise of an accurate and precise thin film.
U.S. Pat. No. 5,618,348 discloses a system for eliminating air from liquid-carrying conduits, such as tubing, included in a system for transfer of a liquid photoresist composition to a pump. There is no disclosure of the conditions under which the photoresist is degassed or coated on a wafer.
U.S. Pat. No. 5,013,586 discloses a process and apparatus for applying liquid photoresist to a spinning surface. A gas is introduced into a chamber above the spinning surface in a manner to effect laminar flow of the gas relative to the spinning surface.
U.S. Pat. Nos. 4,955,992 and 5,509,954 disclose systems for degassing a liquid, which utilize a vacuum.
U.S. Pat. No. 5,358,740 discloses a method and apparatus for low pressure spin coating under air to reduce turbulence above the wafer. However, no consideration or means are provided for delivery of degassed liquid to the chamber, and no means for controlling liquid viscosity are provided or considered. In a reduced pressure environment, liquids will out-gas, thereby forming bubbles within the liquid.
U.S. Pat. No. 4,587,139 discloses a process for coating a rotating disk with a liquid magnetic material containing a volatile solvent. An air barrier is positioned above the disc and coating. A gas having a high kinematic viscosity such as helium is injected into the space between the rotating disk and air barrier while the disk is rotated at high speeds. The use of helium alone for turbulence reduction is expensive compared to a reduced pressure method of turbulence reduction.
A process for making battery electrodes, U.S. Pat. No. 5,547,508, discloses dispensing degassed liquid solution onto a substrate in a vacuum environment. The process is used to prepare thin films of the solution using doctor blades or roll coaters to distribute the liquid over the substrate. In this process the liquid solution is degassed so as to create dense films of the liquid after it is suitably cured. The films are for use as electrodes in a battery device. Doctor blades or roll coaters are the means used to spread the liquid onto a horizontally moving substrate in this invention. Electrodes for batteries do not require the same degree of uniformity in the film thickness, as do films used in semiconductor manufacturing. The use of vacuum in this invention is not necessary to reduce turbulence above the substrate. The process and means of this invention are inadequate for making the highly uniform, particle free, thin films of liquid compositions used in modem semiconductor manufacturing. No action was taken to accurately control the viscosity of the liquid being dispensed onto the substrate in this invention.
What is needed is a process and apparatus for conditioning a liquid composition for use on the spin coating of substrates.
The process of the present invention provides such a conditioned liquid. Said process provides a bubble free, constant viscosity liquid that can assist in the production in more accurate and precise thin films.
The present invention provides a conditioned liquid composition used for spin coating in semiconductor manufacturing. Such composition aids in the reduction of turbulence above a rotating substrate while it also provides the solid material for deposition. The process and apparatus for producing such a conditioned liquid composition removes dissolved gases and bubbles. Such conditioned liquid compositions are characterized as having a viscosity that is substantially the same as it was prior to degassing as well as having a viscosity that does not fluctuate. The result is that predictable, accurate and repeatable volumes of the conditioned liquid composition can be dispensed onto a substrate in a reduced pressure environment for spin-coating.
The present invention provides a process and apparatus for coating a spinning solid surface such as a semiconductor wafer with a liquid composition such as a liquid photoresist or liquid antireflective coating. Typically, these liquid compositions have a viscosity between about 1 and about 150 centipoise (cP), more usually between about 1 and about 100 cP. For example, when utilizing a liquid photoresist composition, the liquid photoresist composition is degassed while maintaining a constant viscosity in a first step either by being subjected to a vacuum for a set time period or by being sparged with a high kinematic gas such as helium. This conditioned liquid composition is then dispensed, via a pumping means, onto a solid flat surface positioned in a chamber that is either air under vacuum or helium gas under vacuum. The solid surface having the photoresist deposited thereon is rotated to spread the photoresist over the entire solid surface. The present invention allows for the liquid composition to spread so the thickness of film is substantially uniform.
In a first aspect of this invention, a process and apparatus is provided for which uses reduced pressure for a pre-determined time and under pre-determined conditions to remove dissolved gases and bubbles to produce a conditioned liquid composition that is degassed and substantially maintains a constant viscosity and solvent fraction. The conditioned liquid composition does not out-gas to form bubbles in dispense lines and nozzle when dispensed into a spin-coating chamber that has a reduced pressure environment. By substantially controlling the pressure of the spin-coat chamber, accurate volumes of conditioned liquid composition can be delivered to the substrate.
In a second aspect of this invention, a high kinematic gas rather than vacuum is used to strip dissolved gases from the liquid composition. The process and apparatus maintain the viscosity and solvent fraction of the liquid during the process. The liquid composition of the present invention does not out-gas to form bubbles in dispense lines and nozzle when dispensed into a reduced pressure environment. By substantially controlling the pressure of the spin-coat chamber, accurate volumes of this embodiment of the conditioned liquid composition can be delivered to the substrate. The high kinematic viscosity gas present in this embodiment of the conditioned liquid composition and any excess from the stripping process can be used to further reduce the turbulence above the wafer during the spin up cycle. This preferable embodiment allows for higher spin speeds to be used, thinner films, and greater uniformity of such thinner films using the process and apparatus of this invention compared to previous means in the art.
In a preferable aspect of the present invention, the high kinematic gas is saturated with the solvent of the liquid composition prior to being introduced into the liquid composition to be conditioned. The benefit of such saturation is to minimize the affect degassing has on the viscosity of the liquid composition.
With respect to uniformity, the present invention provides a film that is uniform over a single wafer and that results in uniformity from wafer to wafer. This means that the deviation limitations required by the industry, in the range of 5-100 angstroms, is not for deviations in the thickness of the photoresist film of a single wafer, rather for all wafers subject to the process.
By removing highly soluble gases from a liquid composition that is also maintained at a substantially constant viscosity and solvent fraction in a first step and injecting a precise volume of this liquid composition onto a solid surface to be spun either in a substantially constant reduced pressure environment containing air or a high kinematic viscosity gas, the formation of bubbles and changes in viscosity in the liquid is minimized and the thickness of the coating produced is substantially uniform.
Lastly, an aspect of this invention is a process for conditioning a liquid composition for use in applying thin films, the process comprising degassing the liquid composition of interest, said liquid composition characterized as having solvent and solid portions and wherein said solid portion is at least partially dissolved in said solvent; and substantially maintaining the viscosity of said liquid during the degassing, said viscosity substantially maintained if the absolute viscosity and the standard deviation of the viscosity of the conditioned liquid composition are within a predetermined range.